A number of processes exist which generate synthesis gases (CO/H.sub.2 mixtures) of varying composition from fossil raw materials, for example, for synthesis of methanol or ammonia. When natural gas is the raw material used, steam reforming is the most widely-used process. This process is used at particularly high capacities to supply synthesis gas for ammonia plants. Natural gas ammonia plants provide the greatest proportion of the world's ammonia capacity. The actual high-pressure ammonia synthesis requires, as a feed gas, an N.sub.2 /H.sub.2 mixture adjusted to be virtually stoichiometric (1:3). An ammonia synthesis plant is generally linked closely, in terms of raw materials and energy, with the steam reformer upstream, which must exclusively produce a hydrogen-rich synthesis gas having a corresponding stoichiometric nitrogen content. The entire process is oriented towards this target. The oxides of carbon (CO, CO.sub.2), which are of necessity, present in various process stages of the reformer as a result of use of natural gas, actually represent incidental constituents which are undesired in terms of the process objective. The existence of such undesired constituents is remedied in that the CO, which is present in the raw synthesis gas after the reforming process, is reacted catalytically with steam to form H.sub.2 in high-temperature and low-temperature conversion. As a result, in terms of the process objective, CO is finally reused in hydrogen or ammonia production. The CO.sub.2, which arises, is a low-energy (low-value) by-product. Some of the CO.sub.2 can frequently be sold as a product, but generates less value on a market, which is generally limited regionally, for example, for the beverages industry. The situation is favorable in ammonia synthesis plant sites, wherein urea synthesis plants, which uses CO.sub.2 as a raw material, are also operating. However, in the majority of ammonia plant sites, sizeable proportions of the CO.sub.2 are generally discharged to atmosphere as surplus, thus, adding to environmental pollution. Because ammonia plants are operated economically only at high tonnages, emission of exceedingly large volumes of CO.sub.2 may result, depending on the plant.
In contrast with other synthesis gas generation processes based on natural gas, the secondary reformer in ammonia steam reformers is fired directly with air for combustion, wherein the nitrogen contained in the air simultaneously introduces into the synthesis gas to be produced, the synthesis component for ammonia synthesis.
In the chemical industry, pure CO is required for the production of ethanoic acid, methyl methacrylates and isocyanates, etc., wherein compliance of its hydrocarbon and hydrogen contents with defined specifications, is a requirement.
A number of processes are known for CO production, their basic structure for reasons of economics being generally oriented specifically towards generating solely CO. Thus, for example, reformers can be used which, in the reformer stage, work towards obtaining the desired CO by means of a CO/H.sub.2 ratio, which is adjusted higher, for example, by means of partial oxidation of natural gas with oxygen. When natural gas undergoes partial oxidation, no steam is used, thus, the amount of hydrogen imported into the synthesis gas is less when used in the ammonia steam reformer. Such a process is described in Berninger (Berninger, R., "Advances in Low-Temperature H.sub.2 /CO Separation"; Linde Reports from Technik und Wissenschaft, 62/88).
In this process, natural gas is converted by partial oxidation with oxygen into a relatively CO-rich CO/H.sub.2 /CO.sub.2 /steam mixture from which the CO.sub.2 and steam are subsequently removed in an adsorber station. The hydrogen purity required is 98%. The CO contained in the hydrogen is condensed out in two stages by applying low temperatures. Alternatively, the gas separation could also be carried out by means of membrane technology or PSA technology (=pressure swing adsorption). The important factor, in order to be able to use the described technology, is that no nitrogen is present in the synthesis gas to be separated.
A further process described in Berninger works with a CO.sub.2 reformer, wherein the natural gas is reacted with CO.sub.2 instead of with steam, resulting in a more carbon-rich (CO-rich) synthesis gas.
Also, in such processes, by returning CO.sub.2 from the process stages, which are installed downstream into the partial oxidation reactor, the proportion of carbon or CO in the product from the partial oxidation reactor is increased, with a view to obtaining a product as rich as possible in CO for the subsequent gas separation.
Depending on the hydrogen required in addition to the CO which is to be produced on the relevant site, it is possible, using the described technologies, to adjust the hydrogen/CO ratio in the synthesis gas within certain limits. However, hydrogen always arises in CO production, and it must frequently also be discharged to the atmosphere or can be used only as a combustible gas and not as a raw material. If pure hydrogen is also to be obtained in addition to the pure CO, it is necessary, for physical reasons, to install an H.sub.2 purification facility downstream, for example using a PSA plant.
In Tindall (Tindall, Crews, "Alternative Technologies to Steam-Methane Reforming"; Hydrocarbon Processing; November 1995, P. 75, et seq.), other alternative processes for steam reforming are described having the objective of H.sub.2 /CO production, steam-methane reforming (SMR), optionally combined with an oxygen secondary reforming stage (SMR/O2R), autothermic reforming (ATR) and thermic partial oxidation (POX). These processes differ in feed gas type and in the use or lack of use of a catalyst, for instance POX, which works without a catalyst. The feature common to all the processes is that they are able to generate both hydrogen and also CO in the form of a mixture or separately, provided that corresponding gas separation processes are installed downstream. When it is desired to produce CO alone, the disadvantage of all of these processes is that only a certain CO/H.sub.2 ratio can be adjusted. When producing CO by itself, one standard reaction to hydrogen always occurs. In addition, a separate single or multistage reformer stage is required.
The H.sub.2 /CO ratio, which is obtainable and is determined by the process, is as follows for the above-mentioned processes: SMR: 3-5; SMR/O2R: 2.5-4; ATR: 1.6-2.65; POX: 1.6-1.8. When subsequent gas separation directed towards CO as the desired product is carried out, there always co-arises a hydrogen-rich fraction of a generally even lower quality. A further disadvantage of the processes having H.sub.2 /CO ratios in the synthesis gas which are in themselves favorable in terms of CO generation is that oxygen, which is costly, must be used as an oxidant. The POX process, which has the most favorable H.sub.2 /CO ratio in terms of CO generation, has the additional disadvantage that soot is produced due to the high partial oxidation temperatures of the natural gas/oxygen mixture, and this reduces the carbon yield, calculated on natural gas used.
The CO/H.sub.2 ratio of the synthesis gas from the reformer plant can also be displaced in the direction of CO by recycling CO.sub.2 from the plant into the reformer or using imported CO.sub.2 in the reformer. However, because of the reaction equilibrium in the reformer, there always remains a hydrogen-rich fraction corresponding to the imported CO.sub.2, which is per se undesirable, and reduces the raw material utilization ratio of the natural gas used as feedstock.
In U.S. Pat. No. 4,836,833, a process is described for separating a synthesis gas derived from a reformer, in which the two target components CO and H.sub.2 are separated through semi-permeable membranes and a PSA plant. This generates a hydrogen fraction of 99 mol. % hydrogen. The CO stream generated simultaneously is only 85 mol. % pure.
The process has the disadvantage that CO purity is inadequate for many chemical processes (for example isocyanate production), thus necessitating the installation of a further working-up stage downstream for the CO fraction. CO can, moreover, never be generated as the sole product.
In EP 291,857, a process to produce carbon monoxide is described in which CO.sub.2 and H.sub.2 are returned into a heat-integrated, reversed water gas conversion reaction in which additional carbon monoxide is generated.
U.S. Pat. No. 5,102,645 describes a CO generation process in which there is generated from a reformer, a more highly concentrated CO fraction in which to carry out a more effective gas separation. The reformer comprises a primary and a secondary reformer. Imported and recycled CO.sub.2 are passed with the hydrocarbon feed into the primary reformer. This primary reaction product is then fed together with oxygen into a secondary reformer, with a carbon monoxide fraction being generated in an autothermic secondary reaction. This fraction has a lower hydrocarbon concentration than that discharged from the primary reformer. The gas, which is returned from the secondary reformer, has a high CO content, such that the subsequent low-temperature gas separation is able to generate a highly pure CO fraction at a lower cost.
Such interventions, in particular, recycling CO.sub.2 into a classic ammonia plant, run counter to the primary objective of generating a hydrogen-rich synthesis gas fraction and run the risk of compromising the operation of the ammonia plant. All of the processes described interfere in some manner with the operation of the reformer, such that continued operation, which is free of disruption is no longer possible under the original operating conditions.
In Lembeck (Lembeck, M.; "The Linde Ammonia Concept (LAC)", Linde Reports from Technik und Wissenschaft; 72/1994), an alternative concept for ammonia generation is described, which differs from the classic ammonia plant. In particular, a secondary reformer fired directly with natural gas and air for combustion is dispensed with and the necessary nitrogen, which is produced separately in an air separating plant and then admixed into the reformer plant for ammonia synthesis is operated completely for hydrogen generation. No CO generation, let alone pure CO generation, is provided in this new ammonia concept.
In DE 4,236,263, a process for generating a high-purity hydrogen stream and a high-purity CO stream from a synthesis gas deriving from a steam reformer is disclosed.
The crux of this process is the generation of a high-purity hydrogen fraction in a PSA plant downstream of the steam reformer, wherein the PSA plant exhaust gas stream is further compressed and is supplied to a multistage membrane separation plant where a pure CO gas stream is obtained.
Disadvantages of the process are that the synthesis gas stream to be processed must not contain nitrogen (as it does in ammonia plant). A further crucial disadvantage in terms of generating high-purity CO is that the pure CO discharged from the membrane separation plant contains virtually all of the methane, which is not acceptable, for example, for isocyanate production. In order to use such a gas for isocyanate production, for example, it is necessary to install a further costly purification step (for example, an additional reformer step) in order to bring down the CH.sub.4 concentration in the CO to the required specification values of&lt;50 ppm CH.sub.4 content. Furthermore, this process, like all the other processes described, necessitates the use of a separate reformer for the CO production and produces a hydrogen fraction for which a use must be found, when the process objective lies in generating CO alone.
Furthermore, coke gasification plants are known for producing pure CO, and these are able to generate a very pure, virtually hydrogen-free, low-methane CO by gasifying coke with CO.sub.2 and oxygen. These processes, however, have the disadvantage of representing obsolete technology with a high level of handling of solids, high costs, and manual labor with potentially considerable working difficulties. Oxygen is moreover, needed for the gasification.
A feature common to all the processes for obtaining pure CO by reforming, partial oxidation with subsequent gas separation (by PSA, low-temperature separation, adsorption, etc.) is the orientation of the primary process specifically towards the requirements of CO generation and the necessity for a separate reformer plant for obtaining CO, and moreover, the tolerance of only low nitrogen contents, which derive from the natural gas.
Hydrogen of generally lower purity arises unavoidably--and this cannot be prevented from the point of view of the requirement for CO alone--and in most cases can be used only for energy or must even be discharged to the atmosphere. Most chemical manufacturing sites, however, already have at their disposal NaCl or HCl electrolysis supplying sufficient high-quality hydrogen for hydrogenation. A further disadvantage of the processes described is that, without the installation of additional fine purification stages, generation of pure CO with the aid of gas separation is at the expense of the purity of the separated hydrogen such that, using such prior art gas separation, the generation of pure CO and pure hydrogen in parallel is either impossible or is possible only at great expense.